This invention relates generally to transmit-receive switching for use with electroacoustic transducers and more particularly to passive networks for accomplishing such switching.
FIG. 1 illustrates a typical electroacoustic transducer array 10 having a common buss driven by a transmitter 14 through a mechanical transmit-receive switch 12. Typically, the noncommon end of each transducer 10 is tied to ground through a shading transformer 18. Individual windings on these shading transformers are connected by beam pattern circuitry to obtain desired beam patterns for signal processing. Across each transformer 18 are shading transformer zener diodes 20 which are intended to breakdown during transmission and limit the voltage and current to each such transformer. When the array of transducer 10 is operating in the transmit mode, the common buss 16 is tied to transmitter 14 through switch 12. In the receive mode, switch 12 grounds the common buss 16 and provides a short circuit across transmitter 14 which is not operating. Thus, in the receive mode, the individual receiving voltage of each transducer element of the array 10 is applied to its respective shading transformer 18.
Networks designed to take the place of mechanical transmit-receive switch 12 have been designed according to the general scheme illustrated in FIG. 2. Transmitter 14 is connected to a series switching element 22. A parallel switching element 24 is connected from the electrical junction of the output of series element 22 with common buss 16 to ground. Series element 22 is generally a plurality of silicon controlled rectifiers in bridge arrangement such that they will breakdown under power generated by transmittter 14 and present a high impedance to isolate the transmitter when it is not operating (in the receive mode).
Parallel switching element 24 on the other hand must present a high impedance at high drive levels during the transmit mode operation and a low impedance for low drive levels during receive mode operation. Switching element 24 must have sufficiently high impedance during transmit mode that transmitter 14 will not be loaded by the switching element in addition to transducers 10. In addition, this parallel switching element 24 must have low enough impedance during receive mode operation so that it will effectively ground common buss 16 at the operating frequency to prevent mutual paths among transducer elements resulting in beam pattern alterations and their associated decrease in side lobe supression.